v] HEREDITY 61 



of allelomorph of a pair it is a ' homozygote,' e.g. 

 individuals of composition TT and tt are 'homozygous' 

 for tallness and shortness respectively. As will be 

 seen immediately, it is possible for an individual to 

 be heterozygous for one pair of allelomorphs and 

 homozygous for another. The essence of Mendel's 

 theory is that owing to the segregation of allelomorphs 

 from each other in the production of the gametes of 

 a heterozygote, the homozygous offspring, when self- 

 fertilised or mated with others of like constitution, 

 breed true to the character in question irrespective 

 of their ancestry. As far as observation can show, 

 the homozygous individuals TT and ft in the genera- 

 tion F. 2 breed as true to tallness or shortness as did 

 their pure-bred grandparents, in spite of the fact 

 that they are the offspring of a cross. 



Hitherto the original parents have been considered 

 as differing from each other in only one pair of alter- 

 native characters (allelomorphs), but Mendel found 

 that in the pea there were several such pairs of 

 characters. For example, some races of peas have 

 purple flowers, others white ; these behave quite 

 similarly to tallness and shortness. The purple 

 flower contains a factor lacking in the white ; when 

 therefore purple is crossed with white, the purple 

 colour is dominant and the heterozygote (F x hybrid) is 

 purple. Such a heterozygous purple if self-fertilised 

 yields 75 per cent, of purple offspring and 25 per cent. 



